1. Field of the Invention
The invention relates generally to modified phenol-formaldehyde resins and products incorporating such resins and, more particularly, the invention relates to the production of a modified phenol-formaldehyde resole useful in the manufacture of molded wood composites with superior surface quality.
2. Description of Related Technology
Synthetic resins, such as phenol-formaldehyde resins, are used as binders in the manufacture of molded wood composites such as man-made fiberboards, e.g. hardboard. Hardboard may be formed in desired shapes and sizes depending on its intended use, for example as a doorfacing or doorskin which is applied to a door body. The principal processes for the manufacture of fiberboards include (a) "wet" processes, (b) "dry" processes, and (c) "wet-dry" processes.
Generally in a wet process, cellulosic fibers (e.g. wood fiber) are blended in a vessel with large amounts of water to form a slurry. The slurry is deposited along with a synthetic resin binder onto a water-pervious support member, such as a fine screen or a Fourdrinier wire, where much of the water is removed to leave a wet mat of cellulosic material having a moisture content of about fifty weight percent. The wet mat is transferred from the pervious support member to a press and consolidated under heat and pressure to form the molded wood composite.
In a dry process, the cellulosic fibers are typically first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, and are then randomly distributed into a mat by air blowing the resin-coated fibers onto a support member. The mat is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated structure.
A wet-dry forming process may be also be used to produce molded wood composites. Generally, in a wet-dry process, a slurry is formed of water, a cellulosic filler, and a resin binder. Sufficient water is then drained from the slurry to form a wet mat. Further water is then removed from the wet mat by drying in order to form a dried mat. The dried mat is then pressed under heat to form the molded wood composite.
Preferably, a wet-dry process begins by blending cellulosic or wood fiber raw material in a vessel with large amounts of water having a pH of less than 7 to form a slurry. This slurry is then blended with the resin binder. The blend is then deposited onto a water-pervious support member, where a large percentage (e.g. 50%) of the water is removed, thereby leaving a wet mat of cellulosic material. This wet mat is then transferred to a drying zone where much of the remaining water is removed. The dried mat is then transferred to a press and consolidated under heat and pressure to form the molded wood composite.
Molded wood composites produced according to the processes described above may, however, have poor surface quality where typical phenolic resins are used as binders. Poor surface quality is indicated where the molded wood composite exhibits poor internal bonding and strength. Further, poor surface quality is also shown where there appear to be layers within the composite which are "flaky" and can be easily peeled away. Where there is poor surface quality, the wood composite may easily break apart and the product is therefore unsatisfactory.
The above-described effects of a poorly bonded surface often result from undesirable pre-curing of the thermosetting phenol-formaldehyde resin prior to full curing of the resin in its final shape in the press. For example, this pre-cure can occur (1) during drying of the wood fiber mat and (2) in the press before the final thickness of the product is achieved.
In order to help prevent the problems associated with poor surface quality when phenolic resins are used as binders, the surface of the molded wood composite may be improved by postpress tempering with drying oils. Many different types of tempering oils may be used, including linseed oil, soybean oil, tung oil, oiticica oil, and unsaturated fatty acid esters. However, the use of these oils increases both cost and production time. In addition, use of such oils is environmentally undesirable. It is therefore desirable to reduce or eliminate the need for such postpress treatment of the molded wood product by preventing pre-cure of the resins utilized in such products.